As is known, the expression high-side switch is used for a switch or a semiconductor switching element whose load path is connected in series with a load between terminals for a positive and a negative supply potential, and one of whose load path connections is connected to the terminal for the positive supply potential. The potential at the other of the two load path connections is in this case dependent on the switching state of the high-side switch, and can rise approximately to the value of the positive supply potential when the high-side switch is switched on.
In the case of a power MOSFET or a power IGBT, the drain-source path through the component forms its load path. When an n-channel power MOSFET or a power IGBT is used as a high-side switch, its drain connection is connected to the terminal for the positive supply potential, so that the drain potential corresponds to the positive supply potential. The source potential of the component is then variable, and depends on the switching state of the component.
With regard to the driving of an n-MOSFET or IGBT such as this which is used as a high-side switch, one problem that arises is that a drive potential must be available at the gate connection which is used as the drive connection, in order to switch the component on, and which drive potential must be greater than the value of the source potential at least by the value of the threshold voltage of the component.
Modern power transistors can be used to switch voltages of up to several 10 kV, so that correspondingly high drive potentials must be available, in each case with respect to the negative supply potential or reference ground potential.
It is known for a driver circuit to be provided in order to produce a drive signal for a high-side switch such as this, with this driver circuit producing a drive signal with respect to the source potential, as a function of an input signal which is supplied to the driver circuit. This input signal can be produced by means of a level shifter from a logic signal which is supplied from a signal processing circuit and is normally related to a reference ground potential. The level shifter is used to convert the logic signal to an input signal for the driver circuit, with this input signal being related to the source potential of the high-side switch.
In order to decouple the high potential which is required at the output of the driver circuit to switch on the high-side switch from the logic potential in the signal processing circuit, it is known, for example from DE 102 05 705 C1 or from DE 10 2004 035 604 B3 for a potential barrier to be provided in the form of a transformer, and for the logic signal to be transmitted to the input of the driver circuit via this potential barrier.
One integrated transformer arrangement, which can be produced in a space-saving form, is described by way of example in DE 102 32 642 A1.
A further problem in the production of a drive signal for a high-side switch is the provision of an adequate voltage supply for the driver for the high-side switch. In this case, an upper supply potential for this driver must assume values above the positive supply potential of the load circuit in order to ensure that the drive signal is produced when the high-side switch is switched on. One possible way to produce this voltage supply is to use a bootstrap circuit, which has been known for a long time, with a bootstrap diode and a bootstrap capacitor. A bootstrap circuit such as this can produce a supply voltage for the high-side driver from a low supply voltage which is related to the negative supply potential.